SWTPC 6800 weirdness
vintagecomputer at bettercomputing.net
Tue Sep 6 17:01:53 CDT 2016
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of william degnan
Sent: Tuesday, September 6, 2016 2:52 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: SWTPC 6800 weirdness
> >You'll need to make electrical measurements, from the system checkout in
> >the manual. You very possibly will have marginal components that need to
> >be replaced, but it's best to try to locate which is bad rather that
> replace at random.
> >A000 is not the same place as 0100. In the 64K space, they're quite
> >Eliminate all but the one RAM board, setting it to 0000. Test that
> thoroughly, then add the next at the next RAM space beyond the first card.
> >Continue until you have enough RAM for a minimal Flex boot. It
> tell you in the version of Flex you're using how much that is (24K?)
> It's hard to do everything at ?>the same time, break it down into chunks..
> Thanks Bill. I've tried working with just single RAM boards, but like
> I said, the only one that will work at all is this modified board. I
> have pics of it here:
> If I could understand better what it is set up to do, what address
> spaces its occupying, I might be able to understand why my 16K DRC
> boards don't work when I try to put them to $A000. I'd prefer to work
> with one of those boards first since the chips are socketed, and then
> I could test the chips individually and be sure one whole board is good.
> I note in one of my pics there, the cap on that modified MP-M looks a
> little tarnished on the outside...
>Getting a memory map of your system is an important step. You need to know what memory addresses each board is attempting to use, so that there is no
>overlap. Also remember that the ROM board has RAM on it too. You would
>not want to map both boards to the same A000 space, but why do you need this at all? What wants free RAM there?
>One important rule is that you don't want to overlap RAM.
>Can you get to a monitor prompt without any RAM installed other than that which is in the ROM board?
Based on what I've read, you *have* to have A000 if your CPU card has been modified for Flex 2.0, which I've verified mine has. When mods on the older MP-A cards are done apparently it disables the onboard RAM, and that's where A000 would be. I could reverse the mod but I'm not sure if I want to forgo FLEX use. So yeah, according to SWTPC.com since that mod was done, I have to have a board at $A000. However, setting either of my configurable ram boards to that space doesn't work. The system will only boot with that weird MP-M in. So there's more to it than that.. probably mods above and beyond.
I suppose it wouldn't be too bad to just reverse the Flex 2.0 mods and start there. I'm doubtful if I'd ever use it and I could always reverse again if I do..
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